The present invention relates to a fuel pump submerged in a fuel tank, and more particularly to a fuel pump for a motor vehicle improved in a pump efficiency.
A conventional fuel pump submerged in a fuel tank is designed to supply a constant amount of fuel to an engine for a motor vehicle even when a fuel surface of a low level in the fuel tank is inclined upon rapid acceleration, cornering or slope running of the motor vehicle.
Such a fuel pump is known from Japanese Patent Publication No. 47-21843, for example. This fuel pump includes first and second centrifugal pumping sections arranged in tandem perpendicularly to a fuel sucking direction. Each of the pumping sections has an impeller adapted to be rotated by a motor shaft. The first pumping section serves to pump the fuel in a main tank to a subtank, and the second pumping section serves to pump the fuel in the subtank to the engine. A discharge amount of the first pumping section is set to be larger than that of the second pumping section. Accordingly, a difference between a fuel amount to be fed from the main tank to the subtank by the first pumping section and a fuel amount to be fed from the subtank to the engine by the second pumping section is stored into the subtank. The subtank has an upper opening communicating with the main tank, so that when a fuel level in the subtank reaches a level of the upper opening, the fuel in the subtank is returned via the upper opening to the main tank. Accordingly, the fuel level in the subtank is always maintained at a constant level. With this construction, even when a fuel level in the main tank is low such that no fuel is temporarily sucked from a fuel inlet hole of the first pumping section, the fuel in the subtank is reliably fed to the engine.
In the above conventional fuel pump, the discharge amount of the first pumping section needs to be larger than that of the second pumping section in order to maintain the constant fuel level in the subtank. However, a discharge pressure of the first pumping section does not need to be so high because the first pumping section is used for the purpose of merely pumping the fuel in the main tank to the subtank. On the other hand, a high discharge pressure is required by the second pumping section because it is used for the purpose of feeding the fuel in the subtank to the engine. Particularly, when the engine is equipped with a fuel injection system, the discharge pressure of the second pumping section is set to normally 2 kg/cm.sup.2 or more.
Generally, a rotating speed of each impeller of the fuel pump is set on the basis of the capacity of the second pumping section because a primary object of the fuel pump is to feed the fuel to the engine. However, since each impeller is rotated by the common motor shaft, the rotating speed of the impeller in the first pumping section is equal to that of the impeller in the second pumping section. As a result, the rotating speed of the impeller in the first pumping section is excessive to cause the generation of noise due to excess discharge pressure and flow.
Further, a high-power engine equipped with a supercharger requires a high pressure and high flow of the fuel pump. However, since this requirement is met by the second pumping section only, the size of the fuel pump or the rotating speed of each impeller must be increased, which causes a reduction in durability, increase in noise and increase in cost. Moreover, as the rotating speed of each impeller is increased, the discharge pressure and flow of the first pumping section become further excessive to result in a reduction in pump efficiency as a whole.